In the standard internal arrangement of a microelectronic acquisition channel reader device, a preamplifier is located at the input to each acquisition channel associated with a sensor that emits an electrical signal. Preamplification enables the power of the electrical signals emitted by the sensors to be increased. As is shown in FIG. 1, a device LECT for reading N acquisition channels C1, C2,  . . . CN that are dedicated to reading N sensors CAP1, CAP2, . . . CAPN is conventionally equipped with N preamplifiers AMP1, AMP2, . . . AMPN, which are all identical from one channel to the next. The input stage for each preamplifier is generally followed by a cascode stage CASC for increasing the open loop gain and increase the dynamic output.
A classic preamplifier comprises one active element, which is generally cascoded. FIG. 2 shows an example of a preamplifier comprising an active element M1 the current of which is polarized by a current source delivering a current Ipol, followed by a cascade stage M2 the voltage of which is polarized by a polarization voltage Vpol, the entire assembly being powered between a first voltage VSS and a second voltage VDD. The signal to be preamplified is applied to active element M1 at an input E0 and exits this element at an output S0 having been amplified.
However, a preamplifier based on a CMOS inverter offers certain benefits over a conventional preamplifier according to the preceding description, in particular its double transconductance capability for the same polarization current, which is advantageous in terms of noise: if the inverter is used, the equivalent input noise is reduced for the same polarization current.
Accordingly, CMOS inverter assemblies are used widely in logic circuits and for analogue applications. The main obstacle to using a CMOS inverter for analogue applications is the control of the polarization current, because unlike logic applications, in which the static polarization current is zero and the transistors are not activated except when a logic state changes, it is necessary to impress a readily controllable polarization current in order to set the transconductance of the PMOS transistor and the NMOS transistor in accordance with the speed and noise characteristics desired.
In analogue applications of such kind, this problem is solved by using a polarization device comprising a voltage regulator with an identical impressed current inverter for reference. Such a polarization device POL is described in a publication by Eric Vittoz: “CMOS et BiCMOS VLSI Design'91” and is represented in FIG. 3. The signal from a sensor CAP that is to be preamplified is applied to a CMOS-type inverter INV at an input E1, and the amplified signal exits at output S1. A voltage regulator REG of which the amplifier stage is installed in parallel with inverter INV is connected to a reference inverter INV_REF that is identical to inverter INV. A current source supplies the impressed current Ipol that flows through this reference inverter INV_REF. Reference inverter INV_REF and voltage regulator REG are powered between a first voltage VDD and a second voltage VSS. As a known current Ipol is passed through reference inverter INV_REF, a reference voltage Vref is obtained at a point B, which is the junction point between reference inverter INV_REF and reference input of voltage regulator REG. If a regulated voltage equal to Vref is applied to inverter INV at a point A, which is the junction point between inverter INV and voltage regulator INV, a current equal to Ipol may be impressed across inverter INV. Moreover, voltage regulator REG serves to ensure low impedance at point A in order to obtain good rejection of variations in supply voltage VDD. Such regulation thus offers one solution for providing effective, stable control of the static polarization current of inverter INV.
The drawback of this solution is related to the unavoidable noise created by the integrated voltage regulator REG as opposed to an external voltage source. In fact, it might be possible to reduce the noise from an external voltage source with the aid of filters with high-value external capacitances, but this is not possible for the noise from an integrated circuit. One obvious solution for reducing the regulation noise is to increase the power that is dissipated into the amplifier element of voltage regulator REG, because if the polarization currents of the noisy active elements are increased, the equivalent input noise thereof is reduced.
The situation thus becomes paradoxical: in order to take advantage of a potential reduction in input noise offered by inverter INV for the same polarization current compared with a conventional preamplifier such as the one shown in FIG. 2, more power must be dissipated into voltage regulator REG.
The noise contributions of the structure that comprises inverter INV and voltage regulator REG, and the noise contributions of a conventional preamplifier constructed with an active element will be compared later in this document. FIG. 4 shows a simplified schematic diagram of FIG. 3 for determining the noise contribution of voltage regulator REG. The intrinsic noise of voltage regulator REG is represented by an equivalent noise source Vb, from which it emanates: VddINV=Vb+Vref.
It should be noted that the noise contributed by reference inverter INV_REF is negligible since it is a simple matter to filter this noise either internally or using an external capacitance. The noise contribution of the reference inverter INV_REF is thus considered to be zero for the remainder of this description.
FIG. 5 shows a small-signal equivalent circuit of the assembly of FIG. 4. The capacitance of sensor CAP of which the signal is preamplified by inverter INV is notated Ce, the capacitances of the N-MOS and P-MOS transistors are notated C1 and C2 respectively, the transconductances of the N-MOS and P-MOS transistors are notated gmINV and the load is notated ZL. id1 and id2 are the illustrations of the noises introduced by the N-MOS and the P-MOS in the form of current sources.
The noise introduced by voltage regulator REG that is referred to the input of inverter INV depends on the impedance of sensor CAP. In the case of a capacitive high impedance internal current source-type sensor CAP, for example a particle (photon, charged particle) detector, the contribution in terms of noise spectral density (in V2/Hz) from voltage regulator REG is:
      V    s    =            V      b        ⁢          Z      L        ⁢          g      MINV        ⁢                            C          e                +                  C          1                -                  C          2                                      C          e                +                  C          1                +                  C          2                    
The equivalent input noise is:
      V    e    =                    V        s                    2        ⁢                                  ⁢                  Z          L                ⁢                  g          mINV                      =                            V          b                2            ⁢                                                  C              e                        +                          C              1                        -                          C              2                                                          C              e                        +                          C              1                        +                          C              2                                      ⁢                  (                      since            ⁢                                                  ⁢            voltage            ⁢                                                  ⁢            gain            ⁢                                                  ⁢            is            ⁢                                                  ⁢            2            ⁢                                                  ⁢                          Z              L                        ⁢                          g              mINV                                )                .            
Using realistic assumptions such as: Ce>>C1, C2, one arrives at:
      V    e    =            V      b      2        4  
On the other hand, the noise attributable to inverter INV is
      V    INV    =            2      ⁢                          ⁢      kTy              g      mINV      (where gmINV is the transconductance of each transconductance of inverter INV) because the spectral density of the noise current of each transistor of inverter INV is Id2=4 kTygmINV.
On the other hand the noise referred to input that is attributable to a conventional preamplifier consisting of an active element ELT passed through by that same polarization current as inverter INV is:
      V    ELT    =            4      ⁢                          ⁢      kTy              g      mINV      
Therefore, in order to retain an advantage over a conventional preamplifier, the following condition must be satisfied:
                              V          b          2                4            +                        2          ⁢                                          ⁢          kTy                          g          mINV                      ≤                  4        ⁢                                  ⁢        kTy                    g        mINV              ,            or      ⁢                          ⁢                        V          b          2                4              ≤                            2          ⁢                                          ⁢          kTy                          g          mINV                    .                          ⁢      Thus        ,            V      b      2        =                  4        ⁢                                  ⁢        kTy                    g        mREG            since gmREG is the transconductance of the active elements that constitute the amplifying element of voltage regulator REG.
Therefore, in order to retain an advantage over a conventional preamplifier, the condition that must be satisfied is:gmINV≦2·gMreg.
The current in the amplifier element of the regulator must therefore be at least equal to that of the inverter in order to satisfy this condition in the typical situation in which said amplifier is based on a differential pair.
It follows that in order to ensure a noise performance at least equal to that obtained with a conventional preamplifier that uses an active element, the use of an inverter assembly with integrated regulation results in a doubling of the dissipated current, that is to say the dissipated power. This represents not an improvement but a worsening of energy efficiency, and the signal-to-noise ratio obtained for a given power is less than that obtained using a conventional preamplifier.